1. Field of the Invention
This invention relates to an improvement in a process for refining crude tetrahydrofuran (THF).
2. Background Art
THF is a commodity in the chemical industry, widely used as a solvent and as an intermediate in the preparation of various polymeric glycols which are useful in preparing polyurethanes.
One of the several methods used to commercially prepare THF is a four-step process which employs acetylene and formaldehyde as starting materials. In step 1 of the process, acetylene and formaldehyde are reacted to form 1,4-butynediol, using a copper-acetylide complex as the catalyst. This reaction is described in U.S. Pat. Nos. 3,560,576 and 3,650,985, both to J. R. Kirchner.
In step 2, butynediol formed in the first step is catalytically hydrogenated to 1,4-butanediol, using Raney nickel as the catalyst. This procedure is described in British Pat. No. 1,242,358.
In step 3, butanediol from step 2 is catalytically dehydrated and cyclized to THF using sulfuric acid as the catalyst, as described in U.S. Pat. No. 3,726,905 to J. S. Coates and V. J. Reilly.
Crude THF produced in step 3 is then refined in step 4, which is a multi-stage distillation, as described in U.S. Pat. No. 4,093,633 to Tanabe, et al.
The THF produced in step 3 of the process contains methacrolein as well as dihydrofurans (2,3- and 2,5-), propionaldehyde and butyraldehydes (normal and isomeric), impurities whose presence causes color formation in polymeric glycols made from the THF. It is difficult to remove methacrolein effectively in the step 4 distillation because of the proximity of its boiling point to that of THF itself. A need therefore exists for a way of separating these impurities from THF easily and inexpensively.